A variety of electrical, electronic and mechanical office equipment must of necessity incorporate suitable ventilators in order to prevent overheating. Such ventilation has been provided for years by small axial flow fans driven by small AC motors, such as shaded pole motors.
In spite of the necessity of their use, fans or ventilators are inconvenient to equipment designers because they make noise and take up space. They are widely used, however, because a wide variety of apparatus and equipment do not work properly without the proper ventilation.
In such environments, fans must meet a number of requirements. These include small size, low cost, minimal maintenance, long life and reliability. Because of these requirements, the smaller "miniature" fans have historically been driven by AC motors, mainly shaded pole types. One reason for this is that such motors meet many of these specifications, particularly low cost and reliability. There are certain conditions that preclude the use of AC fans, and these are specifications which such fan motors do not meet. For example, there are conditions and applications in which a ventilator or fan driven by a DC motor is required simply because AC power is not available. Where only low power is available, DC motors are used because they have low power input requirements. DC motors have an additional distinct and significant advantage over AC motors, namely, variable speed control. For long life, however, a brush-type DC fan motor would be unacceptable because the brushes would wear and require replacement.
It would therefore be highly desirable to provide a brushless motor which could provide the necessary power for driving a fan or ventilator and which would meet the above-stated requirements of small size, low cost, minimum or no maintenance, reliability and low noise.
As indicated above, one of the advantages of the DC motor is the fact that the speed of the motor and the fan driven thereby can be varied. Thus, for example, if the cooling load or requirements are reduced, it would be possible to reduce the speed of the motor thereby reducing the noise produced thereby. The noise of a fan is in part attributed to the speed of rotation as well as the design of the fan itself. For example, if a fan is designed to move sufficient air to provide the necessary cooling at lower speeds, the fan noise would be reduced. However, the power of DC motors is directly related to the input voltage and therefore the speed at which the motor is driven.
One of the deficiencies of DC motors, particularly small DC motors such as those for 3-inch and 41/2 inch axial fans, is the inability to produce sufficient power to provide maximum variability and control capability for the fan to be functional at a variety of air load pressures so the fan can operate at varying speeds to minimize noise problems. The desirability of such DC brushless motors for fan application is well-known as evidenced by articles that have been addressed to the problem, such as an article entitled "Brushless DC Fans" by Ulrich Koletzki published in the Motorcon March 1982 Proceedings, pages 660-672; an article entitled "Brushless DC Air Movers Interface With the Design Engineer" by Fred A. Brown published in the Motorcon March 1982 Proceedings, pages 673-683; and a brochure published by the Panasonic Company entitled "Panaflow by Panasonic--DC Brushless Axial Flow Fan." While these materials discuss the desirability of DC brushless motors and some of the characteristics which such motors should have, the miniature type DC brushless motors for miniature fans have not fully achieved the desired goals which are set forth in the references enumerated above.